Help - intricate problem with internal storage

Tried full wipe of album art cache in power amp. I guess something must have went wrong.
Now, I can not:
-connect to pc with usb (only mass storage works so pc companion can't help me)
-download apps
-when entering file manager it hangs after a few clicks
-gallery doesn't load properly. The app hangs while trying to load content.
- can not enter recovery mode with hard buttons.

My conclusion is there is something wrong with the internal storage.
I do have an sd card but the problems continue with it disconnected.

Factory reset is an option I guess, but I would very much want to backup my messages. If I could get something like a wifi based "backup messages to gmail"-app on there that would help me. But I can't install apps through play.

If I could wipe the whole internal storage and somehow reset it, without resetting the firmware or messages, that would maybe solve it, I don't know.

I'm not a techie kind of guy do please have patience with me! Any help appreciated!