That is very unfortunate.
The reason your .BAK-file fails, is that you have saved twice, then the BAK-file will be identical to the project-file.
.BAK is always the penultimate save.
You should however also be prompted the oppotunity to load a recover-project, unless you deliberately turned that off.
The recover-option is given first time you start LMMS after a crash. Diddent you get that?
There is nothing to do. If both security features did not have success The project is unfortunately lost.
One thing that can lead to data-loss in any program is to use a Labtop running on battery !
and shutting LMMS down before you have seen the message in left corner "Your project has been saved"
Do read about project-security here:
https://docs.lmms.io/user-manual/4-production/4.1
Despite of that bad event, since you are new in Forum
Welcome jizuus !
Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
If you like to introduce yourself, to the community, go here:
http://lmms.io/forum/viewtopic.php?f=4&t=4480