Wouldn't it be pretty gutted manpower- and taxation-wise? And Constantinople would be very close to hostile territory (it'd likely hold out- Theodosian Walls and all that- but raids would be destabilising and bad for revenue).
An unsuccessful Alexios will be potentially deposed-he faced plenty of usurpers IOTL, and here his reputation will be lower. If this happens he will almost certainly be replaced by someone less competent and/or there'll be a round of civil war/coups, further weakening the empire.
A failed first crusade will probably have damaged the Normans, but they'd likely seek to prey on the western bits of the Empire again.