the servo uses vacuum from the engine to work

if there is something in the servo that is missing or broken, it won't work

you should still have brakes, just not without the servo

from reading between the lines here it may be that the master cylinder has had a leak, the brake fluid has leaked down into the servo, the servo is now fucked due to it being contaminated with brake fluid

a new master cylinder seal and a new servo will be what's needed as you can't just strip apart and put a servo back together again as they are air tight

the spring will probably be from behind the brake pedal that gives you the "spring" to pull the pedal back again if it is about 2 inches long and not very tightly coiled

hth