Ford Ka Air Con problem

[DiddlyQuaQua]

Switched air con on last week and had very little performance. Now the air con is switched off but experiencing loud whirring noises from the engine when the engine is started up.
Does the air-con system just need to have it's gas replenished or is the air pump faulty / broken?

[Toneisok]

Have you had a look to see if the drivebelt is slack?, maybe a tensioner pulley has collapsed