Do you have any other Stack or Apache or MySQL installed on the System? You should be able to start/stop/restart the servers using the Windows Manager app unless there are other services in your system using the same ports that MySQL (3306) and Apache (8080) would use.
The default password for PhpMyAdmin is the one you set when installing the WAMP Stack (not Mediawiki). You can reset it by reseting the MySQL password. Everything is explained in the link below:
You don't need to reinstall the Wamp Stack. I recommend you to kill the processes using the Task Manager and then start the servers using the Manager included in the Stack. Regarding how to backup your Mediawiki app, you can backup your database, please take a look at the link below:
I hope this information is useful for you. If you have any other questions, please do not hesitate to let us know.