Is it unethical to not able to complete a journal review within provided time ? I was invited to review a paper from a decent journal in my field. I was provided a month to review the article. 

However, due to some work related commitments, I was not able to provide reviews within the stipulated time (and somehow forgot about the deadline). After 2 weeks of the deadline, I received an email from the editor for providing reviews on the article. I emailed him back that I will provide the reviews and mentioned the date and he acknowledged the same. However, on the date I had provided, I received an automated email that if I don't provide the reviews they will be forced to go ahead with the reviews from other reviewers. 

I emailed them again that I have read the paper and ready with my reviews and will be uploading them within a few hours. But while filling the review form, I received another automated email stating that they have decided to move ahead without my reviews. Since it was my first opportunity to officially review a paper from a journal, the situation left me with the following questions


Was it unethical at my part to not able to provide the reviews within the provided time ?
Will it have any impact on my reputation as a reviewer in future?
Will it have any impact on my reputation as a reviewer in future?


As @darijgrinberg said in a comment, referees are quite often late with reviewing assignments. As an example, on one occasion (the only one I can remember) when I was unable to get a paper reviewed by the deadline I committed to, I sent an apologetic email to the editor a couple of days before the deadline explaining that I will be sending my report a week or two late. His sarcastic reply suggested that he found my apology very amusing and unusual, and he was in fact very pleased that I would be only a week or two late...

So, I think it’s not so much that your reputation will suffer because of this incident; rather, it’s more accurate to say that you have missed an opportunity to set yourself apart from all the other late reviewers - that is, it’s not a negative efffect but the absence of a positive one. But in any case I assume you will have many more opportunities of this type in the future, so I wouldn’t worry about it too much.

As for the ethics, again as Darij said, ethics has to do with willful acts. To forget things or be negligent or put yourself in situations where you are unable to deliver on a promise may be characterized as unprofessional (only a little bit in this particular case) and is certainly undesirable, but this has nothing to do with ethics.