When a job is loaded, many associated Java classes receive an instance. If the instantiation fails, usually because some plugin is removed and this class is no longer available, then the work is hidden.
I suspect that you accidentally deleted another plugin.
Note. Make a full backup before doing anything! The easiest way is to back up the entire Jenkins folder where jobs, configuration, etc. are located.
The easiest solution is to simply install Jenkins from scratch, install the plugins you need (see below for troubleshooting if you are missing some), and then copy the jobs subfolder to the new Jenkins. It’s best to do any configuration under Manage Jenkins manually, but you can also just try copying the related XML configuration files.
If you are missing a plugin and you cannot determine which one, you should look at the jenkins.out.log and jenkins.err.log log files and look for exceptions that occur after Jenkins starts. This may let you know which plugin you are missing.
You can also try editing the job XML files to remove the build steps that you defined from the exceptions (do not forget to back up first!), Then restart Jenkins or select "Update configuration from disk" on the "Manage Jenkins Pages" page.
If you do not dare, but you find relevant views or something else interesting, please update the question with detailed information.