Update from 5.11 to 5.12 totally broke my site - please help

Hello,

First time I'm trying to do an update, and now the site is broken.

I tried to undo and redo the update about 10 times.

I can't restore the database because when I try to import the backup that I made with WAMP I get the error:

No data was received to import. Either no file name was submitted, or the file size exceeded the maximum size permitted by your PHP configuration. See FAQ 1.16.

Lucky for me, I have a working version on my laptop, but again, when I try to export the database from the laptop and import it to my pc, with WAMP, I get this error:
No data was received to import. Either no file name was submitted, or the file size exceeded the maximum size permitted by your PHP configuration. See FAQ 1.16.

I tried exporting as gzipped and as no zip at all.

I tried exporting the whole database and tried to export only the tables and data, nothing works.

The files sizes are very small so it can't be the maximum size issue.

I googled the error but couldn't find any working solution to this problem.

Please, please can someone help me?

Thank you,

Sigal