I have a Casio PCR-T275 that was working fine the last time I used it a month ago and now shows E10 2.00 but none of the function keys work regardless of where I put the key even in th off position.

https://www.manualslib.com/manual/405551/Casio-Pcrt275-Cash-Register-W-15-Depts.html

The cash register might have a water damage on the keyboard maybe you replace the inner keypad cover

Are you sure you mean PLUs and not the department buttons on the keyboard? Which PLU function did you need to change?
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If anyone needs a Casio PCR-T275 cash register manual, c-tech collective have an operation and programming manual here: http://the-checkout-tech.com/manuals/cashregisters/Casio_PCR-T275_users_manual.html

This machine is too basic. it can't have function text changed.
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If anyone needs / looking for a casio PCR-T275 manual, try here:
http://the-checkout-tech.com/manuals/cashregisters/Casio_PCR-T275_users_manual.html

This is a very simple model, so there are only two ways to remove items in a sale. If you enter an incorrect item, press ERR CORR to remove the last item. If you want to remove a previous item, or return an item after a sale, turn the key to RF (refund) mode and key in the item the same as you did in REG mode. This will return it so that it does not affect the daily total. -------------------------------------------------------------------------------
If anyone needs / looking for a Casio PCR-T275 manual, try here: http://the-checkout-tech.com/manuals/cashregisters/Casio3.php

yes u can reset it liken new

The PCR-T275 is a thermal printing register. It does not use an ink ribbon. You simply use 57mm thermal paper and make sure the shiny side is to the front. If you are having printing problems, let me know and I'll try to help. ---------------------------------------------------------------------------------------------------
If anyone needs a Casio PCR-T275 manual, there's one here:
http://the-checkout-tech.com/manuals/cashregisters/Casio3.php

Turn the key to OFF position

have you tried power cycling it??

Hi. Try a flag clear. I think it is in the manual.