If there is a lack of suction it will be because....
Air is getting in where it should not
The hose is blocked
Internal filters are blocked
The motor is not working as it should - unlikely....
If the lack of suction happened when it tumbled down the stairs then it's likely that air is getting in where it should not - carefully examine the case and the hose.
Run it at full power and cover the hose - listen for the sound - when you block up the airway the motor should run slightly faster - a higher pitch.
Do the same with the hose disconnected then you'll know where the air is getting in.
Weather you can fix it depends on the nature of the damage your determination and practical skills.