The few people with cellphone service who had been calling and texting loved ones then called 911 to say that they were trapped in the bank vault and could smell gas.
Before authorities could arrive, people passing by heard them shouting and came to help.
"They started to dig us out and they pulled back some of the rubble just enough for us to open the door and get out," Clark said. "We made it out of the vault and it was just a sense of relief and people were hysterically crying and hyperventilating."
What she saw shocked her.
"It was unbelievable," she said. "I know it was a miracle. Nothing in the bank was standing except for where we were."
They had been in the vault for just under an hour, Clark said.
Clark's cellphone battery was running low, but she managed to text her husband, call her mother and snap a photo of the vault before the phone died.
"I believe I found my car. It's what looks like my car. It was standing vertically against some rubble," Clark said. "It's not a big deal. It can be replaced. Lives can't."
Bank officials expressed a similar sentiment.
"Thank you all for your support as we and many of our members found ourselves in the path of yesterday's storm," the bank posted on its Facebook page. "Sadly, our Moore branch was lost. Fortunately, all employees of that branch emerged unscathed, having ridden out the storm in the safety of the vault."
Hours after the tornado, Clark was reunited with her family and found that her house was relatively unscathed.
Clark said she feels "so lucky and so blessed" to have survived the disaster.
"I wasn't supposed to be at the bank," an emotional Clark said. "It was so clearly orchestrated by God."