Fired from Work. Need advice

So just wanted some advice on what I can do. I got my first job ever at a really good company. I got sick and had 2 days off to recover ( I suffer badly with my bowels and migraines but on top of this I have ear infection in both ears) after those 2 days I was still really bad so decided to go a'n'e and on the way to ane is my work so I popped in to give them an update and they said they understood then Fired me. I know I was on probation and understand that they can do what they please on a probationary period. My only problem is it's been 2 weeks and I haven't been paid. The monthly pay date is the 28th but they said they'd send a cheque in the post the following week ( would have been last Monday 19th) haven't received anything what can I do? I've called up but they've refused to talk