I have architectural shingles. The shingles have separated into two pieces. Can you please tell me what's going on?

When you nail an architectural shingle, there is a small area about 3/4" of an inch wide where the shingles are supposed to be nailed. It is very easy to miss this area. Chances are, that's what happened. There will be a nail line showing where the nails are to be placed. Nails should not vary more than 1/4" from this line.