Restart Mathematica and, before you do anything else, scrape-n-paste this into the notebook

Manipulate[soln=NDSolve[{A'[t]==.08A[t]-12k,A[0]==20000},A,{t,0,5}];
Plot[A[t]/.soln,{t,0,10},PlotRange->{0,25000}],{{k,1},0,500}]

and tap <shift> and <enter> and see if your plot appears.

I don't think that made any substantial changes to your code except for increasing the vertical plot range just a bit so you could see that the plot starts above the top edge of what you had specified.

If all that works then you can start adding back in your plot decorations one at a time and verifying that it works after each tiny change, so you can try to back out if it stops working at any point.

One possible reason for your difficulty is that Mathematica keeps a cache of previous calculations and a prior assignment might have been responsible for you not seeing what you needed. But there is probably no way of verifying that at this point.