Dell U2414H goes into the sleep mode after idling a certain amount of time but it has troubles waking up later. This happens mostly (only?) by using the DisplayPort connector. Googling for the issue shows that this happens with both Nvidia and AMD cards. I suspect that the issue is buggy firmware.
This can be temporarily solved with power-off/on cycle. The power cord must be disconnected but the monitor must not be turned off using the power button. If it's turned off before, then the fix does not help and the monitor stays in the sleep mode.
As far as I know there are no permanent fixes for it. Some people in those forum threads have got their monitors replaced by warranty just to discover that the replacement has the same issue. Fortunately, the issue does not occur always. For me it has happened 2-3 times per month. The monitor is otherwise pretty nice.