1
My phone died.
To sign this, you would sign 'my', 'phone', and then use the sign for 'died'. It is very common and natural in ASL to use this exact sign for electronics losing power, just like we say a battery died in spoken English.
2
The plant died because I forgot to water it.
You can establish the context by signing 'plant', then 'died'. To explain why, you might sign 'because', 'I', 'forget', 'water'. The sign for 'died' works perfectly for all living things, so you can use it for plants and animals just as you would for people.
3
He died last year.
In ASL, time usually comes first. Establish the past tense by signing 'last year' at the beginning of your sentence. Then, point to the person or their established space in your signing area, and finish with the sign for 'died'.