It's best to read a first-hand and also two second-hand accounts of a real red-light spacecraft emergency. SOHO, the Solar and Heliospheric Observatory was almost lost - more than once! It was - and still is, thankfully - in a halo orbit around the Sun-Earth Lagrange or Libration L1 point - about a million miles in front of the Earth towards the sun. This is roughly the same general neighborhood as DSCOVR, where they both keep an eye on the sun.
Briefly, SOHO's orbit was not stable, so it required regular station keeping maneuvers - short burns every few weeks or months's. It's design and orbit were configured so that it would slowly drift towards the Earth, and a sunward burn - perpendicular to its orbit, would put it back in place.
You can read a short, medium, or long version of the exciting story below. I strongly recommend it!
Roberts 2002 is the definitive report and review of the entire situation, problem, and recovery.
In short, you get a hold of some radio telescopes and listen carefully, analyze doppler shifts, and run computer simulations to compare and try to figure out what is happening.
You can see some more information about SOHO's orbit here.