It's most likely they lost one of the primary flight displays, or maybe both primary flight displays through some kind of one in a billion electrical coincidence, and were on the standby attitude indicator, which also provides airspeed and altitude.
If one of the primary flight displays blanked out, whomever has the working display will take over as Pilot Flying, and transfer the Flight Director (and hence the autopilot) to that display if it was linked to the side that failed. With that kind of failure they will certainly request a turn-back immediately, but the return and landing will be pretty routine if one pilot has working instrumentation.
If both displays blanked out for some reason (extremely improbable) they would be on the standby instrument, the little mini attitude indicator in the middle, hand flying, and if they were in the clouds, the stress level would go up quite a lot until they were out of the cloud base and could see the ground, because it's a bit of a challenge to hand fly in IMC with that little attitude indicator in the middle of the panel.
Normally there's a ILS/LOC input to the standby att indicator, so you can fly a "raw data" (raw data means just glideslope and localizer indicator needles, no Flight Director) instrument approach (I've done it in the simulator - takes intense concentration and crew coordination).
If they were visual while all this was happening, the stress level goes way down because in a jet like that, all you really need is airspeed in visual conditions (altitude is nice to have, but you can see the ground so you can more or less maintain a safe altitude and you will already know from experience what pitch attitudes to use, and ATC can correct you if you drift too much up or down).
If you're truly proficient in the type, you should be able to fly in visual conditions without airspeed indication as well, and make it down in one piece, using just pitch attitude and thrust settings you normally set in the various configurations.
For example, flying final approach with full flaps and gear down, the N1 fan speed will always be within some small range, say 66-70% depending on weight. Just set thrust at say 66% if you're light, or 70% in the case of a departure where you're returning still pretty heavy, and follow the glide slope, and the speed will take care of itself, within the ballpark at least, assuming no windshear or other additional excitement.
For really extreme cases where you lose all three airspeed inputs (which is actually a pitot static condition caused by ice crystals at high altitude that block all 3 pitot probes - probability wise, simultaneous triple failures can only come from a single external source) the manufacturers provide pitch/power tables in the cockpit handbook (Quick Reference Handbook) that allow you to achieve ballpark speed profiles on pitch and thrust alone for all the normal configurations and weight ranges. A highly skilled pilot with long experience on type could get by without them, but they cater to the complete range of skill levels.