There's two possible situations with the range: either it goes over a day boundary (e.g. 1800-0600) or it doesn't (e.g. 0600-1800). One of them needs a simple range check and one needs the check given by Mehdi, or a check with the next day. You check which you have by comparing the times:
DateTime now = DateTime.Now;
DateTime start = new DateTime(now.Year, now.Month, now.Day, startRangeHour, startRangeMinute, 0),
end = new DateTime(now.Year, now.Month, now.Day, endRangeHour, endRangeMinute, 0);
if(end < start) end = end.AddDays(1);
if(now > start && now < end){
}