I had an issue where 3 SQLtags stopped updating and displayed the green icon with the question mark. All 3 had the value of null.
The PLC was showing as connected and I found that by altering the address the different value was read then when the original address was put back the tag read as it had been doing for x months with no problem?
Any reason why this should have occured 
I am using Ignition 7.5.1
Secondly, this PLC does get down powered during the week.
What PLC are you using?
You should really upgrade to 7.5.5 since we have made some changes to our drivers.