Is all of their careers over?

Depends on where you thought their careers would go

Josh Radnor hasn't done anything of real note since the show ended.

Cobie Smulders is a part of the MCU so unless she's killed off she's safe for a few years. She had a Netflix show Friends from College that got two seasons. Is about to do another TV show called Stumptown. She's not a superstar but she's still in the public eye.

Neil Patrick Harris mostly works on Stage. Had a successful show on Netflix with A Series of Unfortunate Events. Has done fine post HIMYM

Alyson Hannigan, has gone more into presenting rather then acting.

Jason Segel overall you could say he's been the most successful out of the whole cast. A lot of his best and successful work happened during his run with HIMYM. He's been more quiet over the last few years writing books and staring in more indie fare but overall he's done well.

Like most comedy casts from successful shows one or two strike out and rest don't quite reach the heights of the show. For me Segel, Harris and Smulders have done well.