This is a tip that may seem mundane but has helped me a lot, it is to be self-aware. Sometimes things slip by that we don't notice that could lead to adversely to our mental health.
This may be a toxic friendship/relationship that indirectly was taking a toll on us. This could also be a change in diet/ eating behaviour or exercise routine. It could even be indirectly affected by the work we have or even the tv show we watch. Sometimes it just may be our mental health suddenly went downhill for no reason.
Meditation and journalising can be great tools to aid you through this process to see what could be the root of the problem.