Please make sure you said you lost only Tool ofset data--NOT lost NC ,PLC Data? right.

in this case I never known, But I gusse it is in you NC Program or Workpiecie they are delete them.

If you have 840C "Siemens Operating Manual, I think you can find how to make back up , Tool Data and how automatic load them during power up

Here I have simple way to help my customer "How to back up Tool Data"

#1--go to "Program" --new--New Workpieice --give the name "backupT"

#2--Go to "service"--Manage Data---find the workpiece you made "backupT" --highlight it (not enter) ---> NC--> save start --using select key continue to find Tool Offset --use chnnel 1--back up such as (you have use D1 to D40 ) set the rang what you want, --press start save

the steps may something not exact same, but if i am in fron of machine I will know how to do it, as your refrence

JustinEvans1211

We have an Index G200 with an 840C controller, the hard drive was recently backed up and now every time the machine is powered off and back on the machine loses all of its tool offsets. Unsure about any possible alarms that come up after cycling power. Unsure if anyone has seen this before? 

 Now you provide real situation, your tool data NOT lost, they are overrided by some other values.

see my picture--it is cut from 840C Operator's manaul --if you don't have it , search and download ,

in "Standard Workpiece" the Job list contents Loading commader all data under it will be load during power up you can look for TOA under it youself, or post screenshoot here to show us

my last answer talked to you how to make Tool data back up, you can back up what you want to keep or try to delet TOA file under "standard" ( try to make a copy --paste to another worlpicec )

I think delete is the easy way--

BTW I know some Machine System data cal NCMD can be set for loading what of Workpiece

try my suggestion, If delete not working feedback here I will look for you  

JustinEvans1211

Yes, it is only losing the tool measurements and offsets. The operator was wrong on what it was actually doing, the offsets don't just get wiped out, when the machine is powered back on it loads completely different offset numbers, like it has a setting where it pulls a specific file, not the last used offset numbers. So, I am unsure if it has some kind of wrong file in the hard drive or if it is memory related? Unsure where these numbers get pulled from, I assume the Hard Drive?