As a result, finance director Nick Pomery said, the club will be forced to go into liquidation if it can't raise around £250,000.
The Football League have also docked the U's 10 points, the second club - after Wrexham - to incur the penalty for going into administration.
Pomery told Cambridge's official website: "The club has liabilities of around £900,000 and assets which are in no way sufficient to meet them.
"Our relegation to the Conference means it is not feasible these debts can be paid off by trading profitably.
"We therefore have no choice but to file for administration. We are calculating the amount of money the club will need to present an acceptable offer to the creditors and trade through the close season while we restructure the club to be ready for Conference football.
"We will need everyone's help to raise the money needed, which we expect to come to around £250,000.
"Of this we have already raised almost a third. If the rest of this money cannot be raised the club will close.
"Once the legacy of problems has been resolved and the club comes out of administration, it is the new board's responsibility to ensure that the club operates viably and sensibly in the future.
"We must and will ensure that this situation is never allowed to recur."