3y old boy rat just suddently collapsed

I was away at the hospital for surgery and my husband just called me distraught. Our last remaining rat (the other had to be layed to rest a few months back) just was running around acting terrified/crazy and just suddently collapsed in his water bowl. my husband took him out asap but by that time, mere seconds, he was already gone.

He was fine this morning but by his age I expected it, and my husband felt it too. Now our nest is empty with just my childhood soulcat remaining.