Let it set for a while and then start it up and see if the problem comes back... If it does then most likely a clutch vaccum booster problem... I had similar problems when I first put the engine back in my car... Turned out I had lost one of the check valves for the brake and clutch booster's.... when driving around town, not getting into positive pressure in the intake, the clutch would be Ok at best... but when I got on the highway and went into boost the clutch acted like it wasn't even releasing and wouldn't go into gear...
Also another completely different cause that happened to me was on the original clutch that came with the car. It was as stock disc with a sprung hub... one of the springs came out and some how wasn't letting the clutch disengage. This problem didn't get any better until I pulled the trans and replaced the disc...
