Spring Boot project build failed - cannot find package

I have got a massive problem with my Spring Boot project. For some reason, when starting my application, I get a project build failure error as per screenshot.

It is a huge surprise as I have been developing this app for a while now and obviously launching it number of times without any problems. Today, I wanted to start it again and got that error and yesterday everything worked fine!!! Moreover, I didn't make any modifications to the code or settings.
It cannot find some packages, even though they do exist.
Any help will be very much appreciated!

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.3.2.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>pl.maciejdluzen</groupId>
	<artifactId>hotelreservation</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>hotelreservation</name>
	<description>Hotel reservation system in Spring Boot</description>

	<properties>
		<java.version>11</java.version>
	</properties>

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jpa</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-security</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-thymeleaf</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-validation</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-mail</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-devtools</artifactId>
			<scope>runtime</scope>
			<optional>true</optional>
		</dependency>
		<dependency>
			<groupId>com.h2database</groupId>
			<artifactId>h2</artifactId>
			<scope>runtime</scope>
		</dependency>
		<dependency>
			<groupId>mysql</groupId>
			<artifactId>mysql-connector-java</artifactId>
			<scope>runtime</scope>
		</dependency>
		<dependency>
			<groupId>org.projectlombok</groupId>
			<artifactId>lombok</artifactId>
			<optional>true</optional>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.modelmapper/modelmapper -->
		<dependency>
			<groupId>org.modelmapper</groupId>
			<artifactId>modelmapper</artifactId>
			<version>2.3.8</version>
		</dependency>
		<dependency>
			<groupId>org.thymeleaf.extras</groupId>
			<artifactId>thymeleaf-extras-springsecurity5</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
			<exclusions>
				<exclusion>
					<groupId>org.junit.vintage</groupId>
					<artifactId>junit-vintage-engine</artifactId>
				</exclusion>
			</exclusions>
		</dependency>
		<dependency>
			<groupId>org.springframework.security</groupId>
			<artifactId>spring-security-test</artifactId>
			<scope>test</scope>
		</dependency>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

</project>

Edit 1 - some additional info:

I know that I have got some mess with java versions I've been using as mvn clean install throws the following error (but when I was launching the app using Intellij green arrow button, it was all fine):

Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project hotelreservation: Fatal error compiling: invalid target release
: 11 -> [Help 1]

So in my Maven dependencies I have got Java 11 <java.version>11</java.version>

java -version and javac -version show respectively 1.8 and 9 version.

In the Intellij IDE settings Project SDK is set to Java 14.0.2 and only the following SDKs are available:

java -version

java version "1.8.0_261"
Java(TM) SE Runtime Environment (build 1.8.0_261-b12)

javac - version

javac 9.0.1

mvn -v

Apache Maven 3.6.3 (cecedd343002696d0abb50b32b541b8a6ba2883f)
Maven home: C:\apache-maven-3.6.3\bin\..
Java version: 1.8.0_261, vendor: Oracle Corporation, runtime: C:\Program Files\Java\jdk1.8.0_261\jre

Either install java 11 SDK and add it to Intellij, or change the java verion in the pom.xml to one of the installed version, like:

<properties>
    <java.version>1.8</java.version>
</properties>

Try with fallowing annotation on main class with @SpringBootApplication

@ComponentScan(basePackages = "org.my.pkg")

Have you tried rebuilding the Maven project?