Some of this stuff around the 2.4.x -> to 2.5.x seems to have been communicated pretty poorly, but you can read help/general/light-wallet/developers; lots of people with the same questions you might have.
The gist of it: If your balance is 0, don't panic. Your coins aren't going anywhere, the light wallet just doesn't know about them yet. You just need to generate + attach the same amount of addresses you used on the previous version. This can be time consuming (if you made lots of transactions), in which case there's a script to help with the process:
Check your Tools -> Edit Node Configuration and be sure to set Min Weight Magnitude to 14 if it isn't already.
If you get errors initially about communications issues when attaching: Change your node.
If you get invalid hash issues: Try changing the Curl implementation under Tools -> Edit Node Configuration from Webgl2 to CCurl. Or vice versa.t If it still doesn't work, check that you're on 2.5.1 (and if that's the case, wait for updates).
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