The first question is, what went wrong last fall? If it was properly
prepared and seeded, you shouldn't have to do anything now. Did you
seed it too late in the season so that it didn't have time to get
established? Early Sept is the optimum time. Was the soil tested,
proper PH established, and starter fertilizer applied? Did you use a
high quality seed, preferably endophyte enhanced?
If you just have some small areas, you can reseed those late Mar-early
April, by roughing up the area with a rake, distributing the seed, and
lightly raking again. If there are big areas, then rent a slice
seeder. Spring is a much more difficult time to establish a lawn for
several reasons. Competition from weeds, higher temps requiring more
water and not a lot of time to get established before hot summer
weather arrives are the main ones.
Since your re-seeding, make sure you later put down the proper
crabgrass control, eg Tupersan, which is OK for new lawns.