Moneywise and Yahoo Finance LLC may earn commission or revenue through links in the content below.

Imagine checking a long-forgotten account and discovering it's worth multiple millions of dollars.

It might sound like a dream, or a financial cautionary tale, but that's exactly what happened to Sarah, a 50-year-old mom from Seattle.

Top Picks

• Jeff Bezos backs a platform that lets anyone invest in rental homes for as little as $100 — 6 ways to build wealth like a landlord without actually being one

• A record 45% of central banks plan to grow gold reserves — and many investors are following suit. Get your free gold IRA guide from Priority Gold

• A single line on your car insurance policy could be inflating your premium by up to 30% — here's what to change

Sarah, who said she has homeschooling her children for 20 years, checked in on her employee benefits account from when she worked for a tech giant and made a life-changing discovery. That's when she called into The Ramsey Show (1) to ask for advice.

Sarah's account had gone from worth barely anything to roughly $18 million at its current market price, she told Dave Ramsey. Although she didn't reveal which company it was, some commenters speculated that it could be Nvidia, the tech giant that has surged tremendously during the AI boom.

Regardless of the source of this windfall — and Sarah wasn't sharing those details — this sudden multimillionaire said she had "no idea" what to do with her unexpected boon. Ramsey, as usual, offered some advice.

Diversify and withdraw in a tax-conscious way

To be sure, finding $18M in a dusty account is a huge win, but it also exposes a simple investing risk.

Having so much of your net worth tied up in a single stock is "scary and unwise," Ramsey said. He recommended that Sarah offload some of the shares and invest her money elsewhere. However, given the magnitude of the fortune, selling even a fraction of the account would likely push Sarah into the top tax bracket.

This makes for a tricky situation. Sarah needs to start liquidating her stocks for redistribution, but she needs to be mindful of how quickly she does so.

According to the Internal Revenue Service (2), the highest possible federal capital gains tax rate for someone in this bracket is typically 20% — although there are some exceptions.

Depending on where you live, you may also face state taxes on your capital gains from selling long-term investments. For Sarah, in Washington state (3), that's another 7%.

While Ramsey suggested speaking with an expert tax planner or investment advisor to minimize her tax bill, he was crystal clear on the urgency of the situation. He insisted that Sarah diversify away from a single stock as soon as possible.