I have a implant on a upper tooth, the crown came off. Is it safe to use Dentemp on the crown?

Hi, JK.   
You are so right as to re-cement your crown. The sooner you can get your crown over the implant the better. If you don't the gum tissue will fall over the implant and complicate re-cementing your crown later. This will start within 5 minutes of the crown coming off.  So, place it back on as best you can and then check your bite. Does it feel right to you? If so, you have it on correctly. If it doesn't feel right then re-position the crown until it feels right. You can use denture gel to hold the crown in until you can see your dentist. I have used dentemp before but it is very thick and hard to use. So, don't use that if at all possible.
Denture gel is best. This is the type of gel that is used to hold dentures. Use only a pea size amount.
Hope this helps.

Hi:I personally prefer polygrip or fixodent since it would be easier to remove and easier for you to clean the excess.  However, please go to your dentist right away for proper cementation of the crown.Best,Dr. Maddahi

Things always happen on a holiday weekend!  Don't use Dentemp. You might not be able to seat the crown fully and your bite may be off. I would have you get Poli-Grip and put just a little in the crown and then seat it over the abutment (stump). Before you do that, make sure you can put the crown in properly. Once you know the way to orient the tooth to put it back in, you can put the Poli-Grip in. I hope this helps!