Ok, so, Senators weren't always elected by popular vote. At the beginning Senators were elected by state legislators. The 17th amendment changed that so that they were elected by the people rather than by other politicians.
I'm not going to go so far as to say we should go back to that. There are a lot of good reasons to have senators elected by the people and give people more representation and say in government. However the founders obviously didn't consider the country being run that way. Because of that, I think the 17th amendment has broken some things. Most notably, and most relevant at the moment, impeachment.
As originally designed, the house of representatives were supposed to be the representatives of the people, directly elected, and tasked with bringing their constituents' issues to congress. The Senate, on the other hand, was supposed to be a moderating force. Separated by a step or two from the electorate, they'd have a more free hand to debate, consider, and sometimes make decisions and vote in ways that would make people angry, but (ideally) be for the overall good of the country.
Breaking that separation between them and the electorate has tied the hands of senators. Placing everything they do on the debate floor under the constant vigil of C-SPAN has only made it worse. In the cases of highly partisan, highly publicized issues like impeachment, or judicial confirmations, no matter what Senators may think, or wish to do or say, they're unable to do their jobs of properly of arguing for or against something their party disagrees with. They're unable to even publicly show division from their party in a lot of cases, out of fear of the next election.
Now, in argument for directly elected senators, I must go no further than to point out the quality of character of our politicians and how unpopular they tend to be, even among their own constituents. I wouldn't trust most of them as far as I could throw them, so some manner of being accountable is reasonable, and given our system of government, the only manner of being accountable to the people for anything short of criminal charges is simply to vote them out at the next election. (They can be accountable to their peers in the senate, and the senate can and has exercised that ability to unseat a senator.) Power corrupts, and it's good to have a method, of trying to keep the whole system from becoming so corrupt that it rots from within. But having a good reason for the 17th amendment doesn't change the fact that it inadvertently broke other parts of the governmental process that didn't account for direct election of senators to be a thing.