The best advice I can give is entrusting yourself to somebody, be it:
- crisis line
- your general practitioner
- family
- friends
- self-help groups
Mental America offers a guide which covers different stages and strategies before (e.g. first warning signs) and during a crisis.
When the low point of the crisis passed, you can take precautions to be better braced for the next crisis. Think about activities that elevate your mood up for example. This could be strolls through a park, drinking hot chocolate, taking a hot bath or shower, drawing, reading, etc. Writing a diary can also be helpful. A list with more activities to improve your mood can be found here. Everything is part of self-care.
If you are unconcerned but know someone who currently experiences a crisis, I've found these resources:
Crisis & Recognition
What To Do In A Crisis
How to help in an emotional crisis