I think I might have severe depression but I don't know what to do?

Your Dr can assess. for mood disorder & underlying medical conditions that may be causing symptoms. Talking therapy (Cognitive Behavioral Therapy or insight oriented psychotherapy) is allows for lasting change. Psychiatric meds may be indicated. Don't isolate. Spend time w people. Use your support system. Get out. Get at least 8 hrs of sleep/ night & eat healthfully. Daily aerobic physical activity is a big +. Try >
Seek assistance. If someone thinks they have severe depression, the first step would be to see your primary care physician to rule out any medical cause, then the next step, if the depression is severe would likely be antidepressant medication and psychotherapy - the combination is often more effective than either alone. The good news is that depression is very treatable in the vast majority of the ccases.