is a beta blocker
used in the treatment of hypertdnsion and cardiac .conditions.You are on relatively small dose of meddication , 25 mg is the probably the smallest possible dose as the usual dose is 50 mg.
You can safely take one more tablet if you are not sure whether you have taken the tablet or not, as 50 mg of atenolol ifyou are already taking 25 mg should not be a problem.
All these medications have a wide margin of safety , so you can take one tablet without worrying about any thing.
Hope it helps.