What battery is used in the wrist unit of the Polar F11

this is f11 it may help you
http://timlaqua.com/2008/09/how-to-replace-the-battery-in-a-polar-f11-heart-rate-monitor-hrm/

u may have started wearing a Polar F11 Fitness heart rate monitor to help you keep track of your pulse during cardiovascular exercise. The monitor consists of a strap with electrodes that you attach to your chest, which transmit heart rate data to a receiver worn on your wrist like a watch. The wrist unit stores data and settings that you can customize, such as your weight, height and target heart rates. However, you may wish to reset the unit and start over with new settings. You can reset your Polar F11 by following a few easy steps.
Instructions

1. • 1 Press any button to activate the Polar F11 Fitness heart monitor unit.
     
   • 2 Press all buttons on the Polar F11 Fitness heart monitor simultaneously, except the "Light" button. There are five buttons and the "Light" button is at the top left side of the unit. Continue to hold the buttons until the display fills with digits.
     
   • 3 Press the "OK" button, which is located on the right side of the unit in the middle of the up and down buttons. The Polar F11 Fitness heart monitor will reset.
     

You may have started wearing a Polar F11 Fitness heart rate monitor to help you keep track of your pulse during cardiovascular exercise. The monitor consists of a strap with electrodes that you attach to your chest, which transmit heart rate data to a receiver worn on your wrist like a watch. The wrist unit stores data and settings that you can customize, such as your weight, height and target heart rates. However, you may wish to reset the unit and start over with new settings. You can reset your Polar F11 by following a few easy steps.
Instructions:

Press any button to activate the Polar F11 Fitness heart monitor unit.

• 2 Press all buttons on the Polar F11 Fitness heart monitor simultaneously, except the "Light" button. There are five buttons and the "Light" button is at the top left side of the unit. Continue to hold the buttons until the display fills with digits.
  
• 3 Press the "OK" button, which is located on the right side of the unit in the middle of the up and down buttons. The Polar F11 Fitness heart monitor will reset.
  

This worked for me. From the manual, "If there are no reactions to any buttons, reset the wrist unit by pressing all the buttons simultaneously for two seconds. All other settings but time and date are saved."

Try resetting. From the manual, "if there are no reactions to any buttons, reset the wrist unit by pressing all the buttons simultaneously for two seconds. All other settings but time and date are saved."

Is it possible the watch block is not in correctly and the display is getting squished?

You should have someone who is use to servicing heart rate monitors take a look at it. There are parts in the monitor that can corrode and would need to be cleaned. Also, look into replacing the battery in both units (Strap and wrist) and make sure the unit is reset properly.

try this link for the manual.
polar f11 heart rate monitor manual Google Search .
However, you may wish to reset the unit and start over with new settings. You can resetyour Polar F11 by following a few easy steps. Press any button to activate the Polar F11 Fitness heart monitor unit. Press all buttons on the Polar F11 Fitness heart monitor simultaneously, except the "Light" button.

How to Reset a Polar F11 ' Healthfully

https://healthfully.com/reset-polar-f11-7176556.html reset polar f11 heart rate monitor Google Search

Open Caseback. Unclip the contact. Replace battery, clip contact back and put the caseback on.

Sounds to me as the integrated circuit has been damaged.
In this case you will not be able to repair it yourself as watchmakers skills needed.

try a new battery