*Spoilers* Sort of...
After Caida LIbre, where you shoot the plane down, you are locked out from Michael and Trevor, they are both laying low due to Trevor's actions at the end of that mission.
While locked out, you can only play as Franklin, and do the Deep inside Devin mission (at the D) or the Minor Turbulence mission (at the M at Michael's house, not sure if you have to spend a few days as Franklin for this to appear). As with all missions, you need to walk/drive into the yellow circle at Michael's House.
Once you complete Minor Turbulence, you should be able to switch to Michael and Trevor again. However, they have both lost their LS privileges, courtesy of Martin Madrazzo (the guy whose house you pulled down earlier in the game). If you enter the city as either Trevor or Michael, goons (hired goons) will be after you. You can still enter the city, and it's fairly easy to escape, they won't follow you past the city limits.
Then you do the Paleto Score missions.
Reading some of the posts above, some of you seem to be getting kicked from GTAO, then ending up in the SP game. Have you tried just letting SP load? Random, but it could be a factor.