The toilet was installed under the stairs a year

Take the lid off the cistern tank and have a look. There is a ( usually ) round float ball in there. This is what is causing it. When the cistern fills up the ball is supposed to float up and the arm it attaches to shuts off the water, usually at the opposite end to the float.
Turn the tap on the pipe to the cistern so it is nearly off and move the arm up and down by hand. See what it is getting stuck on or what is happening.

If it is not a simple sticking problem, you may need to undo the top of the cistern valve itself ( opposite end to the ball. )
Turn the water off completely to the cistern to do this or you will get squirted. :)
They are fiddly little valves and all a bit different but if you take your time and make sure you have a good light you will be able to work out what has been happening.
Hope this is some help.