Your plan B can be any of them:

1. Msc in mathematics (2 years course):It will open various private sector jobs for you adding Phd to it will open plenty opportunities for you.

2.B.Ed (2 years course):You will be eligible for teaching after this course

3.MBA(2 years course):It will open Buisness administration opportunities for you.

4,Other government exams like SSC, banking,state government exams as they have similar syllabus and exam pattern as UPSC.

Hope this helps.