So, your TT does no longer give you an "a" when you enter an address or you have to press on weird spots in the menu to select something?
It usually means your digitser for the screen is partially faulty but you might be able to "fix" it without buying a new one.
All current Navcore can use a texfile "cal.txt", located in the root of the device to override the default calibration of the digitiser.
I will base my instruction on that and not on the cilb.txt for older Navcores (little bit different).
Ok, how do I know the values for the cal.txt?
Start your TT (best after a reset) and when the drums start press and hold the power button - this will show you the Linux status screen with all hardware related info.
Row 4 from the top shows your current bootloader, row 4 from the bottom shows the original calibration data of the digitiser.
Only the first values are of interest, the rest shows the voltage for the digitiser.
As an example from my device:


93 921 147 840
The values are:
93 = min X
921 = max X
147 = min Y
840 = max Y


One word of advise: the values inside the cal.txt are in a different order!
The sequence of the cal.txt is : Max X Min X Max Y Min Y
That means the 93 921 147 840 on the status screen translate to 921 93 840 147 inside the cal.txt!
With those values the will react exactly the same way as without the cal.txt present, if that is not the case check the values and the place again.
Keep in mind this is only an example and you have to use the values as shown on your status screen!


Normal touchscreens use a co-ordinate system and a point calibration, TT prefers to calibrate the start and end of the X- and Y- axis, so be prepared for a lot of reboots and changes inside the cal.txt!
I only describe how to calibrate the Y-axis, procedure is identical for the X- axis.
As by my example start changing the mx Y value.
If you lower the value the reaction on the screen should go down, if you increase the value the reaction should go more to the top of the screen.
Change the value by about 50, save the cal.txt in the root of your device and boot the TT.
You should notice a little difference already.
Try to remember by how much it has changed to get an estimate on how much further you need to change this value only.
Once you reach a point were you think it feel ok start the address input with the big keyboard for the finetuning.
Hint: If you notice after the changing the max value there is barely any change adjust it by about 200 and try again, if still no real change use the min value to start the calibration.
You now change both the min and the max value to get the most accurate result, using a touchpen is a good idea.
Only minor changes in the values should made at this stage and only one value at a time, otherwise you might end up messing around.
The final check should confirm the menu buttons work fine, all characters and numbers in the screen keyboard are assigned correctly and the zoom buttons in the map screen are working.
Don't be too disapointed to notice the keyboard works but the zoom buttons don't - they are very small and in the corners so only a perfect calibration will get them back working.

In case the input is off in both axes you should still start with the Y section to get the height correct before adjusting the x axis.
Changing both at the same time makes it really hard to figure out what change caused the reaction on the screen.
I hope this will to get your TT going again.

As a real world example:
The Y axis on tendriver's X20 was out of calibration & had to be lowered. The original Y values, on the Linux status screen, were: 134 856. After re-calibrating the Y axis back into alignment, the new Y axis values were 460 50. These new values were entered into the cal.txt file.
Keep in mind this is only an example, and you have to use the values as shown on your status screen!