|"I have it connected with an HDMI cable."|
I'm assuming that's always what you have been using, and that it's length has not been increased.
RIGHT click on a blank part of your main desktop screen, choose Personalize, then Display settings.
Click on the display you're having problems with.
Change the resolution - pick one that fills the TV's screen properly.
(When you have clicked on the problem display, you're adjusting the resolution for only that display.)
The display on the TV will not change until you have clicked on OK at the bottom of the Display settings window.
The resolution should have the same ratio of width to height as your TV display does.
E.g. if it's 16:9, when you divide the first resolution number by the second one it should be = 1.7777 (to 1)