1) Your friends may have begun to make and spend more money than you, and you cannot participate in their activities. Many people here will say “just find new friends” not realizing how different their ages, lives and skillsets may be from yours. You only have to say no a few times before you are left out. You might even feel bad every time you turn them down.
2) You may be overburdened with work to the point where your most meaningful social connections are at work. Especially if this is a startup, if you take the night off to go drinking you might not deliver on a critical contract which may cause your work friends, who may have in the interim become your only friends, as well as you, to no longer have a place of employment.
Whatever it is I’m sorry you are going through this.