I have had 11 teeth removed and an immediate denture put in but I cannot remove it?

Not 1st day. Dentist should have you back the next day to remove it, adjust it and check the extraction sites. You leave it in to help reduce swelling and protect the extraction sites. I see my patients the next day and 2 days after that and the next week and the next week. In between if needed. Very important to adjust quickly early on.
See your doctor. You may want to visit your doctor and have them instruct you on how to remove and re-insert the denture. Sometimes your doctor will adjust the denture.
Timing. If it has not been 24 hours, I would not try and remove it. If more than 24 hours, I would call the oral surgery/dental office and ask since they are aware of the surgery that you had done.
See your dentist. It is removable, and your dentist will help you learn how to do it yourself.
Post surgery. You should see your dentist that made the denture for a adjustment as soon as possible. I usually see my patients the next day after the surgery.
Extractions when? When were the extractions performed? When you say you cannot remove your immediate denture, is this because your dentist advised you not to, or you physically cannot remove it because of pain, or a tight fitting denture?