If the lever (or round handle) is missing, you should still be able to grab the shaft of the valve itself, the part where the screw goes, and close the valve.
Words of caution here:
1). If you have not used the valve for many years, it may be stuck, so be cautious.
2. If you want to put a handle back on in the future, be careful not to damage the ridges on the valve stem
2). I have also found that often time valves that had say unused for many years, which are then closed, tend to leak from the stem or don't seat off well. So know where your main isolation valve is in case you need to turn it off.
Another option...
Make a trip to the hardware store (the orange or blue one), and take a look at replacement valves. Ask the sales rep to help you find a screw for a similar valve you can take home to replace your lost screw.
Of course if you have lost the handle or lever then this option would be of no use.