How to import trained keras models
I am afraid if you had saved the model in your drive rather google collab directory as it will lost after run-time session interrupt.

model_new = load_model('BestModelWeights.hdf5')  # will load model , if BestModelWeights file  exist in collab directory --which i feel lost in your case that could be the reason you are getting "where it saved" error.

I would advise use google drive to save weights and directly load weight file when you need.For this you need to follow two steps 


Mount google drive and authenticate collab.

from google.colab import drive
drive.mount('/content/drive')
Load model

model_new = load_model('/content/drive/My Drive/BestModelWeights.hdf5')


and you can play with your new loaded Model.

hope it helps.
JSH