Absolutely nightmarish. Honestly, a Trump presidency is a less frightening proposition at this point. Remember, she still will have to contend with an extremely hostile GOP house and Senate. They will give her no quarter.
The GOP house would absolutely refuse to pass anything, even appropriations bills. Endless cycles of government shutdowns and GOP filibusters begin The GOP will prevent the debt ceiling from being raised, our credit rating will suffer as has happened before. Financial markets will be extremely volatile, consumer confidence will collapse. We'll be looking at another recession in short order.
The GOP Senate will refuse any Clinton appointee full stop. Government posts will remain unfilled and slowly the Federal Government will cease to function. No new supreme court nominees, the court will slowly die out.
Clinton's domestic agenda would be stillborn, she'll be unable to accomplish anything. She could attempt to govern by executive order, but these orders are going to get challenged in court every single time.
Eventually, the GOP would submit a politically impossible budget, something Clinton has to veto. The GOP will tie the budget to defunding Planned Parenthood, entitlement cutbacks, or any one of Paul Ryan's wet dreams. The GOP will then look to vulnerable Democrats to form the 2/3 vote needed to defeat Clinton's veto. The GOP knows that deep down, Democrats want to govern and that some type of appropriations bill needs to pass. The government shutdown will be extremely painful for the American people. However, the GOP would gladly watch the country burn if they could rule the ashes. A 2/3 majority is reached and the government is back up and running for a few more months. The cycle will repeat itself many times, each time the GOP another concession from the Democrats.
Republicans will begin a new effort to disenfranchise voters. Under the auspice of protecting the integrity of the vote, new restrictions are put in place at the state level in swing states. "We can't have another 2016" will be the perfect cover to eliminate or reduce early voting, and absentee voting. Voter ID laws start popping up in other states. These restrictions greatly reduce the chances Democrats have in 2018 and 2020.
2018 midterms approach. Democrats have absolutely nothing to show their base. Clinton is unable to accomplish nearly any of her goals. Meanwhile, the GOP attacks with renewed vigor. The election "was stolen" will be their battle cry. GOP turnout will be immense, they'll pick up a few more house seats and possibly more in the Senate. The Alt-Right makes its pretense known similar to the Tea Party caucaus
2020 approaches. Clinton, still unable to see the writing on the wall, still as arrogant as ever, seeks the nomination. The establishment begrudgingly supports her. She loses in a landslide. American voters chose to have functioning government over the divided mess of 2016-2020.