“I cannot seem to get myself to do things. I trudge along. If its the weekend, I slump in front of TV and do nothing all day. Even at work, all I seem to do is stare at my pc screen. My work is suffering as a result too. I cannot think and I feel sad all the time. I just want to crawl under the duvet and stay there.”
It sounds like you might be suffering from depression. If you have been feeling like this for a while, I would recommend talking to your GP or going to a counsellor or psychologist. Depression can take over your life.
If it is over a day or two only, you might be suffering from PMT. Eat some chocolates and curly up with a good book. A nice warm bath helps too.
Whatever you do, try to avoid alcohol or you could feel worse.