Hi @yova ,
Are you sure that the light is red and not amber?
The [https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-gx280_user%27s%20guide_en-us.pdf|User guide] doesn’t mention anything about a ''red light'' but states that if you have an ''amber light'' on startup, to try reseating the memory and any other cards plugged into the motherboard (disconnect the power before opening the computer and touching anything).
In the guide, click on ''the Type of computer > Solving Problems > Power problems'' to view the information.
Also check the ''diagnostic lights'' found on the back of the computer to see if they are showing a colour pattern which may help in determining what is going on.
In the User guide, click on ''Type of computer > Troubleshooting Tools and Utilities > Diagnostic Lights'' to view what the pattern means (if there is one that is).
Hopefully this is of some help