The reasons for this could be many, but if I had to guess, it's probably some custom Javascript that is causing the problem. 

Before you open the task form in IE, press F12 to open the Developer's Console (note: If you're opening from an email link, then just press F12 once the browser opens, and simply refresh the page). 

You should automatically be on the Console Tab (as shown): 

If you are running into any errors, they should be displayed there with the error symbol (). 

If you are unable to correct the errors on your own, please take a screenshot of them or copy / paste the error text into your reply so that we can get a better idea as to what is happening. 

If you have any custom javascript that you suspect might be causing a problem, but are unable to correct, please also post that along with your reply. 

I hope that this helps you. 

EDIT: I noticed that you created a second topic which states that your tabbed form isn't working in IE11. Another thing that might be happening is the forced emulation of a lower browser version than the one you think is currently running. I have written about how to possibly fix this here.

Looking at the picture below. What value does it say in the highlighted area when you look at the Developer's Console on your own Browser?