For the first time in decades, the Metropolitan Transportation Authority had been on the verge of having as much money as it needed to run and repair the continent’s biggest transit system.

That changed on Wednesday, when Gov. Kathy Hochul said abruptly that she would suspend a tolling plan that would have raised money the authority needed for critical improvements to New York City’s subways and buses.

The authority is now suddenly contending with a gap of $15 billion in its capital plans, throwing into question whether it will be able to maintain the century-old infrastructure that millions of New Yorkers rely on by repairing and upgrading aging equipment, modernizing signals and technology, and making subway stations more accessible for people with disabilities.

The governor could replace the money, at least temporarily, with funding from the state’s reserves. She is also said to be considering proposing a tax on New York City businesses, which would require the approval of the State Legislature. That is far from guaranteed, especially with just one day left in the legislative session.

Ms. Hochul’s decision has put the M.T.A. back in the familiar position of having to jockey against other state interests in order to fill its coffers.

“Where it leaves the governor is in between a rock and a hard place, because she’s the one that said that she had the money,” said Lisa Daglian, executive director of the authority’s Permanent Citizens Advisory Committee, a watchdog group. “It’s going to be very surprising to very many of us as to where that money is going to come from,” she added.