saya punya form pesan ke admin, tapi ngga masuk kedatabase, apakah ada yang salah?

<form action="<?php echo base_url(); ?>hubungi/kirim" method="post" onSubmit="return validasi(this)" id="commentform"  class="comment-form">
									<div class="clearfix"></div>
									<div class="comment-form-input-wrap td-form-author">
										<input class="" id="author" name="a" placeholder="Name:*" type="text" value="" size="50" aria-required="true">
										<div class="td-warning-author">Please enter your name here</div>
									</div>
									<div class="comment-form-input-wrap td-form-email">
										<input class="" id="email" name="b" placeholder="Email:*" type="text" value="" size="50" aria-required="true">
										<div class="td-warning-email-error">You have entered an incorrect email address!</div>
										<div class="td-warning-email">Please enter your email address here</div>
									</div>
									<div class="comment-form-input-wrap td-form-comment">
										<textarea placeholder="Your Message:" id="comment" name="c" cols="50" rows="10" aria-required="true"></textarea>
										<div class="td-warning-comment">Please enter your Message!</div>
									</div>

									<p class="form-submit">
										<br> <input name="submit" type="submit" id="submit" class="submit" value="Send Message">
									</p>
								</form>