I think one of the first keys to moving on is talking to someone who you trust to listen. I can imagine a number of scenarios that can fit into the rather generic question you've put forth. Unfortunately, not everything can be solved in the same way, so lacking details to understand what you're struggling with means there's not really much advice that can be given to help. Find a friend, a loved one or a professional and open up to them. If you're not used to that, go with a professional. While they're paid to listen, they're training to be as unbiased as possible and that can make you feel less vulnerable.

My blanket approach is to get a hair cut (I do it rarely), take a few personal days to reflect (often trying to avoid speaking aloud entirely, so I can work on my inner dialog), cry a bit (if warranted) and have a few drinks a trusted friend. Cutting my hair is a way for me to mark a fresh start, so it's mostly just ceremonial.

Then try and find something to lift you out of your funk. Change career paths. Learn a new skill. Find something exciting.