D&D 5E What are the rules for interrupting a spell with a casting time longer than one action?

[ECMO3]

For example, if I am casting mending (1 minute) or prayer of healing (10 minutes) or planar binding (1 hour) and someone casts Power Word Stun on me, does that end the spell and I need to start over, or does it just resume when I am no longer stunned? Is it the same for silence or

Bonus question - can I use a reaction to cast a spell while I am casting another spell (so in the example above can I counterspell the Power Word Stun while in the middle of my other casting)?

 

[CleverNickName]

I used to take a hard line to spell interruption: if you were interrupted while casting a spell, or couldn't complete it in the time allotted, it just failed and you lost your spell slot. Sure, it's fair...and it's balanced...and it's probably what the game developers intended.

But it's boring. Nowadays, I just ask the player. "Oh no, your spell was interrupted! Did something dangerous and unpredictable happen, or did it fizzle out harmlessly?"

If they say 'dangerous and unpredictable,' I roll on the Wild Magic table. Otherwise, I let them describe what it looks like when the spell fizzles.

 

[Swarmkeeper]

For example, if I am casting mending (1 minute) or prayer of healing (10 minutes) or planar binding (1 hour) and someone casts Power Word Stun on me, does that end the spell and I need to start over, or does it just resume when I am no longer stunned? Is it the same for silence or

Longer casting times (more than 1 action) require concentration. Being Stunned results in the incapacitated condition which breaks concentration.

Here's the bit from the PHB:

Longer Casting Times p202​

Certain spells (including spells cast as rituals) require more time to cast: minutes or even hours. When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so (see "Concentration" below). If your concentration is broken, the spell fails, but you don't expend a spell slot. If you want to try casting the spell again, you must start over.

Bonus question - can I use a reaction to cast a spell while I am casting another spell (so in the example above can I counterspell the Power Word Stun while in the middle of my other casting)?

This is a tricky one. I'm inclined to allow it as long as the reaction spell does not require concentration (none do as far as I'm aware).

 

[WyrmworksDale]

I don't think you can cast 2 spells at a time, so if you're mid-cast, you can't cast another until you finish the first.

 

[Seramus]

@Swarmkeeper is on the money. It takes your concentration, which can be broken in the usual manner, but does not expend the slot.