Открыл файл .Jar как блокнот.И теперь все файлы .Jar открываются как блокнот.Что делать,и как вернуть всё обратно?

Откройте в нужной программе.

.jar файл это файл архива для Java. Java не является обязательной частью Windows 7.

Это может быть установленная Вами среда IDE наподобие NetBeans, эмулятор телефона Андроид или какая-нибудь игрушка.

https://docs.oracle.com/javase/8/docs/technotes/guides/jar/jarGuide.html

What is JAR?

JAR stands for Java ARchive. It's a file format based on the popular ZIP file format and is used for aggregating many files into one. Although JAR can be used as a general archiving tool, the primary motivation for its development was so that Java applets and their requisite components (.class files, images and sounds) can be downloaded to a browser in a single HTTP transaction, rather than opening a new connection for each piece. This greatly improves the speed with which an applet can be loaded onto a web page and begin functioning. The JAR format also supports compression, which reduces the size of the file and improves download time still further. Additionally, individual entries in a JAR file may be digitally signed by the applet author to authenticate their origin.

Executable JAR Files

On Microsoft Windows systems, the Java 2 Runtime Environment's installation program will register a default association for JAR files so that double-clicking a JAR file on the desktop will automatically run it with javaw -jar. Dependent extensions bundled with the application will also be loaded automatically. This feature makes the end-user runtime environment easier to use on Microsoft Windows systems.