It sounds like your battery is weak.  I found with my '86 Vanagon that
the fuel injection would not work if the voltage, measured at the
battery while the starter was turning, was less than 10.25 vdc the
engine would not crank.  If the voltage was greater than 10.50 volts
while the starter was turning the engine would crank.  The starter would
turn over the engine at a pretty good rate even at the <10.25 volts level.

Try measuring your voltage at the battery posts with a cheap digital
voltmeter while you try to crank the engine.  If it cranks then there is
no problem.  If it doesn't crank then it is easy to measure the
voltage.  If it is less than 10.5 vdc then this is probably your problem.

Also, try cranking the engine with a booster battery.  If it cranks OK
then buy a new battery.

Good luck,
Ed McLean