I doubt that they have gone bad but being in your fridge theres a risk of cross contamination from other pro-biotic sources. That will make for a bad tasting brew.
If you try to use them again, you will know for sure when smelling your first batch of kefir if they are good or not.
It’s really not a big deal since buying pure kefir grains on-line cost less then 20.00 USD.
If you got 20$ just order some on-line.