XC holds are a relatively common occurence, and more so now that frangible pins are increasingly being used in fence construction. It is worth having a plan in your head for the time that it happens to you.
Firstly don’t panic about your time. The fence judge will have noted the time that you passed their marker, before you were stopped, and will also note your re-start time.
Obviously what you do depends on the length of the hold – your fence judge should be able to give you an idea whether it’s likely to be a couple of minutes for a fence repair, or a long wait for treatment of an injured rider or horse. Most times it’s best just to walk the horse quietly on a long rein. If you know you are in for a long wait you can get off and loosen the girth etc. At the bigger events there is even water provided at stopping points so you can sponge the horse down. You are allowed assistance eg. someone to hold/walk the horse for you.
Thankfully long waits are rare, although I was once held at the far end of the course for nearly an hour when the rider in front of me broke their leg in a fall. The fence judge kept suggesting that I dismount, especially when they landed the air ambulance, but knowing my horse I felt far safer remaining in the saddle and letting him mooch around whilst I chatted to people and raided their picnics!
When it comes to be time to restart you will be given plenty of notice, especially after a long hold. Make sure you get your horse’s mind focussed on the job again, and he’s warmed up and off the leg if you’ve been stopped for a while. Most people don’t know that you are allowed to jump a fence from another (lower) class to get you going again, providing it doesn’t have crossed flags on it, and that you still jump all the fences on your course in the correct order. When I was stopped for an hour it was right in front of a pretty tricky water complex and they allowed me to go back and retake the (straightforward) fence before so that I didn’t have to jump the water ‘cold’.
Check where the timing point is – your restart time will be taken from here, so you need to pass it at XC speed.
Most of all – keep cool and don’t panic. A mid-course breather may even work in your favour!