Here's a process I would try to follow (apologies up front for the slightly 
sarcastic tone):
Unless you're Chuck Norris (see #7)- Make a backup copy ... no make that two ... one in a separate folder to experiment on ... and another one tucked away in a safe place ....of your presumably broken Metabase H2 database. (It's probably named
metabase.db.mv.db) - perform all following steps on that first experiment copy of the file. - "Install H2" - meaning place a copy of
h2-<something>.jar- from http://www.h2database.com in your experiment folder. - Try to see if e.g. other experiences already shared in this forum works -
e.g. a quick search turned up: Unable to connect to Metabase h2 DB - #2 by eduavencama - If the previous step didn't work, try your luck with the H2 recover tool
- depending on the outcome of the previous step (which I haven't tried myself, because I don't currently own a broken H2 database) try gently salvaging - either by starting a copy of
metabase.jarin your experiment folder on the repaired H2 database or otherwise manually ... extract/dump whatever can be extracted. - Now go and revise and follow your procedures regarding backup - including making sure that you can actually work with data you restore.
- Share your experiences here in http://discourse.metabase.com