what means "attempt call nil

i made program and it worked then i changed the code. it didn't work so i changed it back and now it says: Attempt call nil
please help.

pass"password"

print("Reactor Control Console")
write"Password:"
input=read("*")
if pass==input then
print("Password Correct!")
sleep(2)
term.clear()
term.setCursorPos(1,1)
print("Reactor Control Console")
sleep(10)
x=5
while x>0 do
rs.setOutput("back",true)
print("Reactor is Active!")
sleep(30)
rs.setOutput("back",false)
print("Reactor is in Cooldown mode!")
sleep(30)
print("180seconds cooldown left.")
sleep(30)
print("150seconds cooldown left.")
sleep(30)
print("120seconds cooldown left.")
sleep(30)
print("90seconds cooldown left.")
sleep(30)
print("60seconds cooldown left.")
sleep(30)
print("30seconds cooldown left.")
sleep(30)
print("Cooldown is Complete!")
sleep(2)
term.clear()
term.setCursorPos(1,1)
print("Reactor Control Console")
x=x-1
end
print("Program is finished, rebooting")
sleep(2)
os.reboot()
else
print("Password Incorrect!")
sleep(2)
os.reboot()
end


program is saved to startup file. and previously it worked but not anymore… now it says:"Attempt to call nil"

pass"password" 
print("Reactor Control Console") 
write"Password:"

Should be:

pass = "password" 
print("Reactor Control Console") 
write("Password:")

Attempt to call nil means you are trying to use/call something that is not defined yet/no value set

thanks. it's working again