All apps crash all the time I have a Sony Xperia U which worked fine. I did not change anything lately. 

Today all apps crash on startup. Even Play Store, com.google.process.gapps and android.process.acore. The media gallery seems empty too. Whenever I start something, it crashes, except for opening system settings.

When I connect my phone with my computer the internal storage seems completly empty - so I can't backup anything. 

Is there anything I can do to recover my phone and to keep my data like pictures, contacts, ...? *

I tried the save mode, but the erros occur there too.
I tried to boot in Recovery Mode, but all button combinations did not work. I also tried in USB debugging mode using ADB, but that did not work too. 

* Since Play Store is not working I can't install any backup tools.
Try booting into recovery and backup there if you have an option. If not, then wipe it there or with the key combination: Volume Up+Home+Power
Source: www.phonetipsandtricks.com/tips/sony-xperia-u/hard-reset-factory-reset-14/