The company confirmed an "issue" in a post on X, saying that it's aware of the problem impacting some customers.
Many people are being stuck in "SOS" mode on their phones, unable to send messages or place calls.
Verizon said it's working on the issue, but it was not immediately clear what was causing it or how many users were affected.
A spokesperson said it began at 9 a.m. EST.
By 1:30 p.m. EST, the website downdetector.com was tracking more than 66,000 outage reports.
After 5 p.m. EST, the spokesperson said engineers are making progress on the network issue, and some service has started to be restored.
"This morning, I was in Indiana at the time, realized I had no data, couldn't open GPS. So, I had no idea where I was going. Fortunately, my friend had data," Vu Nguyen said.
Investigators from the Federal Communications Commission are looking into the outage.
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Customers in the Chicago area and Indianapolis appear to be the hardest hit.
The FCC is already investigating a similar outage with rival network AT&T back in February that impacted more than 70,000 customers.
Verizon's outage happened hours after the company announced a $3 billion plan for a third-party company to lease and manage its cell towers.
"We know how much people rely on Verizon and apologize for any inconvenience some of our customers experienced today. We continue to work around the clock to fully resolve this issue," Verizon said in a statement.